1. Field of the Invention
The present invention relates to a ceramic circuit board for a semiconductor device, and to a method for manufacturing this circuit board, and to a high-output module.
Semiconductor elements include LD (laser diode or semiconductor laser), APD (avalanche photodiode), and other such optical semiconductor elements; HEMT (high electron mobility transistor), HBT (hetero-bipolar transistor), and other such semiconductor elements using GaAs, InP, Si/SiGe, or the like that can operate at high speed; IGBT (insulated gate bipolar transistor) and other such inverter/power converter silicon devices; and BiTe and other such thermoelectric semiconductor elements, and the circuit boards used in these fields need to have low electrical resistance, good thermal radiation, well-matched thermal expansion, and a super-fine wiring pattern for higher integration and speed.
2. Description of the Prior Art
A conventional circuit board will be described through reference to FIGS. 4A-4F. As shown in FIGS. 4A-4E the process has been as follows up to now. A metal mask or photomask 2 is applied on a ceramic substrate 1 (FIG. 4A), a first metal layer 3 is formed by vapor deposition or sputtering, and the metal mask or photomask 2 is removed (FIG. 4B), after which a resist 4 is formed (FIG. 4C), and then a second metal layer 5 is formed by vapor deposition or sputtering (FIG. 4D), and the resist is removed to obtain a completed product (FIG. 4E).
The ceramic substrate 1 is made from AlN or alumina. This is disclosed, for instance, in Japanese Patent Publication 2-271585. The first metal layer is used for a resistor, and TaN, NiCr, or tungsten is generally used therein. The second metal layer is used for a wiring or an inductance, and has a laminate structure comprising Ti/Mo/Au, Ti/Pt/Au, Cr/Mo/Au, or Ti/V/Au. The reason for using titanium or chromium for the layer in contact with the ceramic substrate is to increase the adhesion strength to the substrate. Because the platinum, molybdenum, or vanadium in the middle has a high melting point, it is inserted in order to prevent the top layer from alloying with the metal, i.e., titanium or chromium used in the above contact portion. Gold is used for the top layer, and is selected in order to successfully perform wire bonding or die bonding. An example of the combination of materials in the completed product is shown in FIG. 4F.
With a substrate for a power semiconductor, copper or gold is applied to the entire top surface of a ceramic substrate by vapor deposition, plating, or fusion, after which a wiring pattern is formed by etching.
To produce a high-output module, semiconductor elements are mounted on these circuit boards by means of die bonding.
With today""s high-output modules, in addition to making the modules smaller merely to reduce the size of the final device, there is also the need to make the wiring patterns much finer with a reduced size so that higher frequencies can be handled. It is also necessary to lower the resistance of the metal portion of the wiring in order to reduce loss of high frequency characteristics and decrease power consumption, and to this end thick-film techniques have become necessary to increase the thickness of the wiring patterns.
To satisfy both of these requirements at the same time, it was necessary for the thickness of the metal layer used for wiring to be at least 5 xcexcm, and for the aspect ratio (D/L) between the wiring thickness D (xcexcm) and the distance L (xcexcm) between the adjacent wiring pattern lines to be D/L greater than 0.4, but a conventional circuit board could not be processed so that both of these could be satisfied.
The reason for this is that a fine pattern could not be formed on a substrate, onto which a thick film resist has been applied, with a vapor deposition process relying on a metal mask or photomask, which is a conventionally practiced fine wiring process, and that vapor deposition had to be continued for a long time in order to obtain a thick film, so practical application was difficult. Also, when a wiring pattern was formed by etching, it was difficult to perform fine processing of a pattern smaller than the wiring thickness because side etching occurred, and etching removal was particularly difficult. Consequently, a miniaturized high-performance high-output module could not be realized.
It is an object of the present invention to provide a circuit board having thick-film fine wiring patterns, and to realize a miniature high-performance high-output module.
In order to solve the above problems, the present invention is constituted as follows.
(1) A circuit board comprising a first metal layer formed in patterns on a ceramic substrate, a second metal layer formed in patterns on the first metal layer, and a third metal layer formed covering the entire top surface of the second metal layer and the majority of the side surfaces of the same, wherein the first metal layer and the partial second metal layer not covered by the third metal layer are reduced in width by etching.
(2) The circuit board according to (1) above, wherein the combined thickness D (xcexcm) of the first, second, and third metal layers and the distance L (xcexcm) between adjacent pattern lines satisfy the relationship of the following formula.
D/L greater than 0.4 
(3) The circuit board according to (1) or (2) above, wherein the combined thickness D xcexcm of the first, second, and third metal layers is at least 5 xcexcm.
(4) The circuit board according to any of (1) to (3) above, wherein the second metal layer includes at least one selected from the group consisting of copper, nickel, silver, and aluminum.
(5) The circuit board according to any of (1) to (4) above, wherein the outermost layer of the third metal layer is gold.
(6) The circuit board according to any of (1) to (5) above, wherein the ceramic substrate contains at least one selected from the group consisting of alumina, AlN, and Si3N4 in an amount of at least 90 wt %.
(7) The circuit board according to any of (1) to (5) above, wherein the ceramic substrate is diamond or cBN.
(8) A method for manufacturing a circuit board comprising:
vapor depositing or sputtering a first metal layer on a ceramic substrate;
forming a resist in patterns;
applying a second metal layer on the first metal layer by plating using the resist as a mask;
making the resist into a thin layer;
applying a third metal layer on the top surface of the second metal layer and the majority of the side surfaces of the second metal layer by plating; and
removing the resist and then etching the first metal layer so that the first and partial second metal layers not covered by the third metal layer are reduced in width by etching.
(9) A high-output module, wherein at least one high-output semiconductor element that generates a heat of at least 10 mW is joined on the circuit board according to any of (1) to (7) above via solder or an electrically conductive resin.